The present invention relates to a method of manufacturing a wire screen product and to a wire screen product intended particularly for the screening of fiber suspensions in the wood processing industry.
The wood processing industry uses two basic types of to screening drums. One type is a drum made of a metal plate in which screen apertures, either holes or slots, have been manufactured with a desired spacing while the plate is planar after which the plate has been bent cylindrical and the edges have been welded together to form a cylinder. During the past two decades screen plates having certain kinds of grooves machined thereto before manufacture of the screen apertures have become very popular. In a finished screen drum the grooves are located parallel with the axis of the drum and the screen apertures are located at the bottom of the grooves.
The other basic type of screen drums, the so-called wire screen drums, are usually manufactured by securing support wires to a cylindrical jig onto which the screen wire is wound up with a certain pitch from a reel by rotating the jig. The screen wire supplied from the reel in secured to the support wires by welding. When a wire surface of the desired size has been formed the cylinder having the screen wires on the outer side and the support wires on the inner side is detached from the jig. After this the cylinder is cut open in its axial direction and bent to form a planar surface and further, the planar surface is bent in the opposite direction to form a cylinder so that the screen wires extend essentially in the axial direction and the support wires are parallel with the frame.
EP-A1 -0 182 688 discloses a screen drum made of wires arranged parallel to the screen drum axis and supporting rings arranged on the outflow side of the screen drum in a plane perpendicular to the screen drum axis. Both the wires and the supporting rings are provided with grooves for matching and fastening the two components together.
EP-A1-0 432 448 discusses a wire screen drum where the wires have a profiled cross-section and where the supporting rings are round of their cross-section. The wires have been fastened to the supporting rings by means of welding. For ensuring that the weld surface with its irregularities does not catch fibers passing by the weld seams have been covered by soldering.
E-650 690 discloses a screen for sorting wood chips coming from a chipper. The screening members have been fastened to each other by means of grooves substantially in a similar manner to EP-A-0 182 688.
DE-A1 - 42 24 727 discusses a screen drum having axial wires fastened to supporting rings. The wires have been secured to the supporting ring by means of pressure welding.
FIG. 1 illustrates a conventional prior art method of securing the screen wires 2 to a support wire 4. The securing is done by welding (W1, W2) the support wire 4 at its both sides to the screen wire 2 for example bad the MIG or TIG welding method. In practice the securing method described above has been proved to be inadequate because very often the pulses stressing the screen wires gradually fatigue the joint and thus the screen wire will gradually come off from the support wire. It should be noted that, in addition to the pulse-like radial forces, the wire is stressed also by the radial force of the rotating movement of the fiber suspension which tends to turn the screen wire around its securing point. Naturally, a failure of the welded joint results in bending of the screen wire/wires, i.e. local bulging of the screen towards the rotor which brings about a risk of the screen wires hitting the rotor and being cut which in turn destroys at least the capacity of the screen to clean pulp. In the worst case, pieces of metal detach from the wires which cause, when proceeding with the accept, unpredictable damage in subsequent pulp treatment stages, not to mention the fact that only one screen wire detached from its securing weld causes a distinct change for the worse in the purity of the accept because the size of the screening aperture has changed at the detaching point of the screen wire.
FIG. 2 illustrates a second prior art way of securing the screen wires 2 to the support wire 4. In that figures the edge 6 of the support wire facing the screen wire is sharp. This form of the support wire 4 is particularly advantageous when resistance welding is used during which the welding current malts the edge 6 and the support wire 4 is partly pressed into the back surface of the screen wire 2 producing a substantially central welding point Wc. It is also possible that the parts of the support wire and the screen wire to be pressed against each other are sharp, i.e. the main configuration of the cross section of both the wires is triangular whereby the edges of the wires are pressed against each other and partly inside each other during welding when the wire material fuses.
Practice has, however, shown that irrespective of the shape of screen and/or support wire, the single resistance welding point Wc is not adequate to keep the wires reliably affixed to each other but in the end the wires will be detached in the same way as wires welded at sides.
On the other hand, both wire screens and screens manufactured of plates share some drawbacks. It has been known for decades that screening of cellulose pulps is based on causing the fiber suspension to be screened to rotate. This rotating movement, or rather the speed difference, is created either by rotating the fiber suspension along the screen surface by a particular rotor, or by rotating the screen drum in relation to the practically stationary fiber suspension. It is also a typical feature of the screen apparatus mentioned that the real flow direction of the suspension is axial, i.e. the suspension to be treated is supplied to the apparatus at one end of the screen drum whereby at least at the beginning of the screening process while the accept yield is the strongest the flow is predominantly axial before the rotating movement of the rotor or the drum turns the flow to resemble a spiral with a decreasing pitch. It is typical of the operation of the apparatus described above, or rather of the behavior of the suspension in the apparatus, that in most cases the flow direction of the untreated suspension approaching the screen surface is axial although the suspension in some cases is fed in tangentially but at such a low rate that the rotation velocity of either the rotor or the screen drum is clearly higher. Then, when being influenced by the rotating means of the apparatus, i.e. either the screen drum or the rotors the flow direction of the suspension turns more and more parallel with the periphery, or in practice the suspension flow assumes the shape of a spiral having a decreasing pitch towards the discharge end.
According to the old screening theory, it is essential that the screening apertures, particularly slots (when they are used) are substantially perpendicular to the flow. Also many different ways of manufacturing the above screen plate apertures are known. The apertures may have the shape of round holes or elongated blots. The manufacture of a so-called slot screen will be described here. The slots are usually manufactured by willing a plate-like basic material so that a wider so-called background groove is milled at first and after hat a narrower slot is machined through the plate in the groove either on the background groove side or on the untouched side of the plate.
The machining tool in both these stages is a narrow milling cutter giving a fairly long bevel area at the end of the groove/slot. This kind of a manufacturing method may well be used also in the manufacture of the so-called PROFILE screen plate (a design developed by A. AHLSTROM CORPORATION, today owned by CAE Screen Plates, Inc.) the surface of which facing the pulp to be treated has been provided with grooves to improve the screening efficiency of the plate. A characteristic feature of the screen plates manufactured in this way is that the narrow slot is in the apparatus itself on the side of the apparatus facing the pulp to be treated whereas the background groove is in a screen on the so-called accept side and in a thickener on the filtrate side.
Also laser cutting and so-called electron beam (cutting have recently been introduced for cutting narrow slots. By these methods the cutting is performed practically perpendicular to the surface of the plate and the background grooves are necessarily not needed.
On the other hand, the industry employs so-called wire screens in which the screen cylinder comprises a large number of adjacent wires with screen slots between them. The wires have been secured to each other in one way or another on the side opposite to the pulp to be treated. If the wires have been arranged extending substantially in the axial direction of the drum, in practice a slot having the length of the whole drums is formed which is interrupted only by the support wires or corresponding means disposed on the xe2x80x9cbacksidexe2x80x9d, i.e. on the accept space side of the drum. A screen drum of this type has been found to operate remarkably well in certain conditions. For example, In plants where utmost purity of the pulp is not required but the capacity is the most important factor, wire screens of this type have proved to be excellent, particularly in the screening of dilute pulp with very narrow slots.
It has been concluded that the high capacity is due to the fact that the flow passes through the screen along the slots, in other words the flow direction remains axial and the flow drifts smoothly through the screen because there are no discontinuity spots in the screen, i.e. the slot does not seem to be interrupted at all along the whole length of the screen. Correspondingly also impurity particles have time to adopt the correct orientation to end up in the accept which reduces the purity degree of the accept.
In milled screen drums, the bracket neck between the slots which follow one another axially breaks off the flow particularly in the so-called inlet end of the screen cylinder, in which the flow still is practically axial, and thus prevents smooth flow through the screen surface. Further, also in milled drums there are dead spaces in the slots/grooves which reduce the efficient area of the slot. These factors reduce the capacity of the screen plate/drum but improve the purity of the accept obtained, because the brackets between the slots interrupt the path of the impurity particle gliding in the groove and bounce the impurity off from the vicinity of the groove and thus the impurity particle does not have time to find the right alignment to pass through the slot.
It has been noticed generally that most of the screen capacity is obtained from the top third of the dry, i.e. from the third receiving the suspension to be treated. This has been explained to result from the fact that in that portion of the drum the suspension flow is to a large extent parallel with the slots or deviates only little from it; thus, the theory presented above is confirmed. The further into the screening space the suspension proceeds, the longer time the rotor has accelerated the velocity of the suspension and the higher the velocity component of the suspension in the peripheral direction has grown. In other words, the pulp moves in the lower end or the discharge end of the drum already almost in the peripheral direction and only a small portion of the pulp fibers pass into the screen slots.
As the industry pursues to obtain as good purity and at the same time also as high capacity as possible it is desirable to try to combine the advantages and to avoid the disadvantages of both of these screens.
The object of the method and apparatus of the present invention is to overcome the difficulties in strength and precision of prior art wire screen cylinders. This object will be reached by employing both a manufacturing technique of the wire screen and a form of the support wire or the screen wire used in the manufacture which allow manufacture in two steps so that the first step comprises securing the screen wire to the support wire by button spot welding and that the second step comprises either a second button spot welding or some other welding method.
Other characteristic features of the method and the apparatus of the invention become apparent from the appended patent claims.